1. Field of the Invention
This invention relates to a method and apparatus for forming a protective film on a disc-shaped recording medium.
2. Description of the Related Art
The photomagnetic recording system is such a recording system in which a recording layer formed of a magnetic material is partially raised in its temperature to higher than the Curie point or a temperature compensation point to reduce the coercivity, and in which a recording magnetic field is applied from outside for inverting the direction of magnetization of the recording layer to record information signals. This photomagnetic recording system is put to practical use in an optical filing system, an external storage device for a computer or in a device for recording the audio or video information.
The magneto-optical disc for recording by the above-described photomagnetic recording system may be exemplified by a magneto-optical disc having a recording layer of a thin magnetic film formed on a transparent substrate including plastics, such as polycarbonate, or glass. This recording layer is made up of a magnetic layer for recording information signals thereon, a dielectric film and a recording layer. The magnetic film is a thin magnetic film made up of, for example, a rare earth-transition metal alloy amorphous thin film having an easy axis perpendicular to the film surface and a large photomagnetic effect. On the reflective film, layered on the uppermost layer of the recording layer, a protective film formed of an ultra-violet ray curable resin, is usually deposited for preventing corrosion or damage to the recording layer.
As the magneto-optical disc, there is also a magneto-optical disc of a double-plate structure, in addition to the above-described single-plate type structure. With the magneto-optical disc of a double-plate structure, two magneto-optical discs are bonded together so that the recording layer sides or the substrate sides face each other. With the magneto-optical disc of the double-plate structure, since signals are recorded independently on the respective recording layers of the discs, the recording capacity is twice that of the magneto-optical disc of the single-plate structure. Moreover, the magneto-optical disc of the double-plate structure is of a symmetrical structure relative to the bonding surface, and hence has a merit that the substrate is less liable to be warped or otherwise deformed against changes in temperature or humidity than the magneto-optical disc of the single-plate structure.
The photomagnetic recording system for recording on the magneto-optical disc is roughly classified into an optical modulation system of recording signals by modulating the light and a magnetic field modulation system of recording signals by modulating the recording magnetic field.
Of these, the magnetic field modulation system is a system of recording signals on the recording layer by inverting the recording magnetic field at a high speed during signal recording in a state of illuminating the light. Researches in this system are going on energetically since this system enables facilitated overwriting as well as high recording density and high speed accessing.
In this magnetic field modulation system, a magnetic field is applied during signal recording/reproduction by a magnetic head which generates the magnetic field in the recording layer. Since this magnetic head needs to be inverted speedily during signal recording, the magnetic field as strong as that of the above-described optical modulation system cannot be generated.
The magnitude of the magnetic field applied to the magneto-optical disc by the magnetic head is inversely proportionate to the distance between the magneto-optical disc and the magnetic head. That is, the magnitude of the magnetic field applied to the magneto-optical disc becomes smaller as the distance between the magneto-optical disc and the magnetic head becomes larger. Therefore, with the magnetic head, the distance between it and the magneto-optical disc needs to be reduced in effecting signal recording.
With the magneto-optical disc of the single-plate structure, this problem is addressed by providing an optical pickup device for illuminating the laser light on one surface of the magneto-optical disc and by providing the magnetic head on the opposite surface of the magneto-optical disc.
For recording/reproducing the magneto-optical disc of the double-plate structure, the optical pickup device is unified with the magnetic head and the optical pickup device and the magnetic head thus unified together are arranged on both sides of the magneto-optical disc.
With this unified optical pickup device--magnetic head system, the laser light is directly illuminated on the recording layer, without interposition of the substrate. Thus, the material of the substrate of the magneto-optical disc may be opaque without having to be transparent.
Therefore, the magneto-optical disc of the double-plate structure has an additional merit that A1, for example, may be used as the substrate material to prevent the substrate from warping. The magneto-optical disc may be classified into the single-plate structure and the double-plate structure. On these magneto-optical discs, protective films are usually formed for preventing corrosion of the recording layer. This protective film is usually formed by the spin coating method.
For forming the protective film, a disc substrate 20, having the protective film formed thereon, is set on the turntable, and rotated at a reduced speed by a spindle motor. A UV curable resin 21 is supplied in a toroidal pattern along an inner peripheral area 20a of the recording layer of the disc substrate 20. The UV curable resin 21 is applied up to the outer rim by the centrifugal force produced by rotating the disc substrate 20 at an elevated speed for coating the entire surface of the disc substrate 20 with the UV curable resin 21. For obtaining a sufficient protective effect after illumination of the UV rays during the subsequent step, this thickness of the protective film is formed to approximately 15 .mu.m.
However, the protective film, formed by supplying the UV curable resin 21 from the inner rim 20a of the disc substrate 20 as described above, tends to be thicker in film thickness as the outer rim if the disc substrate 20 is approached.
However, the protective film formed by the spin coating is varied with viscosity of the UV curable resin, rpm of the magneto-optical disc or the rotating time. However, if the protective film is applied from the inner rim 20a of the disc substrate 20 and the rotating velocity as well as the rotating time is changed for coating the entire surface of the disc substrate to a uniform film thickness, the disc substrate becomes thicker in film thickness at an outer rim than at the inner rim 20a of the disc substrate 20.
FIG. 2 shows the film thickness distribution when the protective film is formed with the viscosity of the UV curable resin being varied from 500 cps through 140 to 37 cps. In FIG. 2, the ordinate and the abscissa denote the film thickness of the protective film and the radial position on the disc substrate 20, respectively. Also, in FIG. 2, characteristics A denote the relation between the film thickness of a protective film formed by a UV curable resin of 500 cps in viscosity and the radial position on the disc substrate 20, while characteristics B and C denote the relation between the film thickness of a protective film formed by a UV curable resin of 140 cps in viscosity and the radial position on the disc substrate 20 and that between the film thickness of a protective film formed by a UV curable resin of 37 cps in viscosity and the radial position on the magneto-optical disc, respectively.
In measuring the relation between the film thickness and the radial position on the disc substrate, shown in FIG. 2, the UV curable resin was supplied in a toroidal pattern at a radial position of approximately 17 mm from the center of the disc substrate 20 rotated at a low speed, the number of revolutions was raised over about 1 second up to approximately 3000 rpm which was kept for approximately 8 seconds, and the UV rays were illuminated to form a protective film, which was the object of measurement. In FIG. 2, a solid line indicates calculated values of the film thickness distribution as found from an equilibrium equation between viscous resistance and the centrifugal force.
In a known manner, the film thickness of the protective film formed by coating the UV curable resin of different viscosities under the same conditions is proportionate to the square root of viscosity. Thus, if a UV curable resin of 140 cps in viscosity and another UV curable resin 500 cps in viscosity are coated under the same conditions, the ratio of film thicknesses of the two films is theoretically (500/140).sup.1/2 =1.9. On the other hand, if a UV curable resin of 37 cps in viscosity and another UV curable resin 500 cps in viscosity are coated under the same conditions, the ratio of film thicknesses of the two films is theoretically (500/37).sup.1/2 =3.7.
Based on the above-described film thickness ratio, a first normalized film thickness distribution was found by multiplying the film thickness distribution of the protective film, formed by the UV curable resin of 140 cps in viscosity, with 1.9. Similarly, a second normalized film thickness distribution was found by multiplying the film thickness distribution of the protective film, formed by the UV curable resin of 37 cps in viscosity, with 3.7. These two distributions are shown in FIG. 3 along with the film thickness distribution of the protective film formed by the UV curable resin of 500 cps in viscosity. In FIG. 3, the film thickness distribution of the protective film for 500 cps of viscosity is indicated by .smallcircle., while the first normalized film thickness distribution and the second normalized film thickness distribution are indicated by .DELTA. and .quadrature., respectively.
In FIG. 3, the first film thickness distribution, second film thickness distribution and the film thickness distribution of the protective film formed by the UV curable resin with the viscosity of 500 cps are substantially on the same curve. Thus it is seen that the film thickness of a protective film is proportionate to the square root of the viscosity on the condition that the same condition is used.
It is also seen from FIG. 3 that if, with the conventional spin coating method used for forming the protective coating, a protective film is to be formed to a pre-set film thickness, film thickness errors of a pre-set value are produced between the film thicknesses formed at an inner rim 20a and at an outer rim of the disc substrate 20, even though operating conditions, such as rotating time and rotating velocity, are varied.
It should be noted that the film thickness difference at the inner and outer rims of the disc substrate can be reduced by using a UV curable resin of low viscosity, increasing the rotating time of the disc substrate 20 and by reducing the film thickness of the coated UV curable resin over the entire surface of the disc substrate 20. For example, by using the UV curable resin of 37 cps in viscosity, lengthening the rotating time and reducing the entire film thickness of the protective film, the film thickness difference of the order of 1.5 .mu.m can be realized between the positions of 24 mm and 40 mm from the center of the magneto-optical disc.
However, if the film thickness is drastically reduced, it becomes impossible to prevent corrosion of the recording layer of the magneto-optical disc. Therefore, the film thickness of the order of 15 .mu.m on an average is required at the minimum over the entire surface of the magneto-optical disc. If the average film thickness of the protective film on the entire surface of the magneto-optical disc is set to approximately 15 .mu.m, the film thickness difference of approximately 5 .mu.m is produced between the inner and outer rims of the disc.
Meanwhile, if, in an optical pickup device adapted for recording/reproducing signals by illuminating the laser light from the side of the protective film, a film thickness difference of the protective film is produced between the inner and outer rims of the disc substrate 20, there is presented a problem that the laser light is subjected to wavefront aberration. If, in the optical pickup device, the wavefront aberration is produced due to the film thickness difference of the protective film, the illuminated laser light becomes unstable to affect the recording/reproducing characteristics. This wavefront aberration W.sub.40d is given by the equation (1): ##EQU1## where n is the refractive index of the protective film, .DELTA.d is the film thickness distribution of the protective film and NA is the numerical aperture of the lens.
In the case of the current magneto-optical disc, the refractive index n of the protective film is 1.58, the wavelength .lambda. of the length of the optical system is 780 nm and the numerical aperture NA is 0.5. Under these conditions, if the film thickness error .DELTA.d is 5 .mu.m, the wavefront aberration W.sub.40d is calculated from the equation (1) to be 0.19 .lambda. (0.148 .mu.m).
Meanwhile, in the optical system of the magneto-optical disc, the wavelength .lambda. of the laser light of the optical system is shortened, whereas the numerical aperture NA of the lens is increased, in keeping up with the recent tendency towards using a higher recording density. This is due to the fact that the spot diameter of the laser light condensed by the objective lens is proportionate to the wavelength .lambda. of the laser light and inversely proportionate to the numerical aperture NA of the lens. If, with the current laser spot diameter of approximately 1.6 .mu.m, the wavelength of the laser light is 480 nm and the numerical aperture NA of the lens is 0.9, the laser spot diameter is 0.5 .mu.m, so that the laser spot diameter can be made equal to approximately one-third the current value. Therefore, with this optical system, the surface recording density can be made equal to approximately 9 times the currently valid surface recording density.
However, if, in case the optical system is modified for improving the recording density as described above, a film thickness error of the order of 5 .mu.m is produced in the protective film formed by a protective film generating device as described above, the wavefront aberration W.sub.40d calculated by the above equation (1) assumes a large value to render it difficult to achieve stable recording/reproduction. If the wavelength .lambda. of the laser light is 480 nm and the numerical aperture NA is 0.9, the wavefront aberration W.sub.40d becomes very large to render it impossible to effect stable recording/reproduction. For suppressing the wavefront aberration W.sub.40d in the optical system of the magneto-optical disc to a value of the order of 0.19 .lambda. equal to the currently adopted value, the film thickness difference .DELTA.d between the inner and outer rims of the protective film needs to be suppressed to not larger than 2.9 .mu.m.
The above-described film thickness difference of the protective film affects not only the illumination of the laser light by the above-described optical system, but also the application of the magnetic field by the magnetic head. The magnetic head used with the above-described magnetic field modulation system may be enumerated by a magnetic head adapted for recording signals with a small gap of the order of several to tens of micrometers from the protective film of the magneto-optical disc and a magnetic head adapted for recording signals as it has a sliding contact with the protective film of the magneto-optical disc. If, in the magnetic field modulation system employing the magnetic head, the protective film undergoes the film thickness difference, the separation between the magnetic head and the magneto-optical disc is changed. This means that, if there is the film thickness difference in the protective film in the magnetic field modulation system, the strength of the magnetic field undergoes fluctuations to reflect this film thickness difference. The result is that the strength of the magnetic field applied to the recording layer undergoes fluctuations. That is, if there is the film thickness difference in the protective film, the magnetic field applied to the recording layer is varied in magnitude during recording of the information signals to render the magnetic field applied across the entire surface of the magneto-optical disc non-uniform. If the amount of float of the magnetic head during signal recording is 5 .mu.m, and the film thickness of the protective film is thicker by 5 .mu.m at the outer rim portion than that at the inner rim portion, the magnitude of the magnetic field applied to the magnetic field applied to the recording layer differs by approximately 15 Oe between the inner and outer rim portions of the disc.
Among the methods of suppressing the above-described film thickness difference of the protective film, there is such a method in which, while the magneto-optical disc is rotated at a rpm exceeding 1000, an ultraviolet beam is illuminated on the disc for forming the protective film. However, if the protective film is formed in this manner, a film thickness difference is produced between the inner rim 20a and the outer rim of the disc substrate 20, with the film thickness becoming larger at the outer rim portion. This film thickness difference is produced by application of the viscous resistance and the centrifugal force to the UV curable resin by rotation of the disc substrate 20 during spin coating. This UV curable resin, supplied to the inner rim 20a of the disc substrate 20, migrates towards its outer rim portion. If the UV light is illuminated from the outer rim towards the inner rim 20a of the disc substrate 20, the resin is cured at the outer rim portion. However, at the inner rim portion 20a not illuminated with the UV light, the resin migrates towards the outer rim portion under the centrifugal force so that it is gradually decreased in thickness. The outer rim of the disc, where the resin is cured on irradiation with the UV light, is gradually increased in thickness because the UV curable resin 21 is supplied from the inner rim 20a where it is not yet cured. Thus, the film thickness difference between the inner and outer rims of the magneto-optical disc becomes larger than if the resin is cured without rotating the magneto-optical disc.